1. Field of the Invention
This invention generally relates to a linear motion rolling contact guide unit suitable for use in various machining tools, precision processing machines and test apparatuses or the like, and, in particular, to a linear motion guide unit having a long stroke.
2. Description of the Prior Art
A linear motion rolling contact guide unit or simply LM guide is well known, and it generally includes a guide rail, a slider and a plurality of rolling members interposed between the rail and the slider to thereby allow to provide a relative motion between the rail and the slider along the rail. Such a linear motion guide unit may be used for guiding a relatively large load over a relatively long distance, and a typical example of such a linear motion guide unit having a long stroke is, for example, disclosed in the Japanese Utility Model Laid-open Pub. No. 61-97626, and another such example is shown in FIG. 4 of this application.
In the linear motion guide unit shown in FIG. 4, a pair of guide rails B and C extending straight is fixedly mounted on a bed A in parallel with sliders D and E slidably mounted on the guide rails B and C, respectively, and a slide table F is fixedly mounted on the sliders D and E bridging therebetween. Any desired object, such as a part of a robot, may be fixedly attached to the slide table F. The slide table F may move linearly in either direction back and forth as indicated by the double arrow G. Each of the guide rails B and C is formed with a pair of guide grooves H and I on its opposite side surfaces extending in parallel with the longitudinal axis of the rail. Each of the sliders D and E is formed with an endless circulating path, including a load path section, a return path section and a pair of curved connecting path sections, each connecting the corresponding ends of the load and return path sections, and a plurality of rolling members, such as balls or rollers, are provided in the endless circulating path. The load path section of the endless circulating path is located opposite to and spaced apart from the corresponding guide groove of the guide rail to thereby define a guide channel therebetween, so that those rolling members located in the guide channel are partly received in the corresponding guide groove. As a result, when a load is applied to the slide table F, it is absorbed by the rolling members interposed between the guide rails B and C and the sliders D and E. For this purpose, the guide grooves H and I and the rolling members are designed such that a proper preload is applied to the rolling members when assembled.
However, if there is an error in the parallel arrangement between the guide rails B and C, the sliding resistance of the slide table F varies as it slidingly moves along the guide rails B and C, which could shorten the life time and could significantly degrade the operational accuracy and performance. In order to avoid such disadvantages, the guide rails B and C must be mounted with a high positional accuracy, which then tends to push up the cost. Besides, if the guide rails B and C are relatively long, such as a few meters long, then it is almost impossible to mount these guide rails B and C with a required high accuracy on the order of a few microns over their entire lengths.
In addition, since the linear motion guide unit is assembled with preloading as described before, as shown in FIGS. 5 and 6, it is necessary to measure widths W1 and W2 of inner and outer guide grooves M and N of a guide rail J and a slider K, respectively, and then to select balls L having a diameter slightly larger than the difference between the measured widths W1 and W2 before assembling so as to provide a desired preloading between the balls L and the guide rail J and the slider K when assembled. However, in this case, since the direction of transmission of force, which is defined by connecting contact points a, b, c and d between the rolling member L and the guide grooves M and N and the center e of the rolling member L and which corresponds to the direction of elastic deformation of the guide grooves M and N and the rolling members L, is a direction which differs from the direction for measusring widths W1 and W2 and which does not allow to make direct measurements, it is extremely difficult and almost impossible to carry out a proper management of preloading.
Also in the case of the structure shown in the before-mentioned Japanese Utility Model Laid-open Pub. No. 61-97626 as shown in FIG. 7, the straight lines defined by connecting the contact points between the rolling memmber P and the guide grooves S and Q and the center of the rolling member P and by connecting the contact points between the rolling member O and the guide grooves T and R and the center of the rolling member O extend in directions which have nothing to do with the direction for measuring widths W1 and W2. Thus, in this case also, it is impossible to directly measure the amount of elastic deformation of various parts of interest.